Most of our current knowledge of mesenchymal stem cell (MSC) biology derives from adult tissue sources. The developmental aspects of MSC biology remain relatively unknown. Yet it is likely that understanding the ontogeny of MSC like populations and their role in normal development can provide important insights toward the postnatal identification, functional characterization, and ultimately the clinical utilization of MSCs. The long-term objective of this application is to apply insights gained from the isolation, characterization, and analysis of prenatal mesenchymal progenitor populations toward the goal of developing clinically useful postnatal MSC populations. The specific aims of this application are: 1) To complete the in vitro characterization of fetal multipotent stromal progenitor populations in the murine model. We have isolated and begun to characterize unique populations of murine fetal multipotent stromal progenitor (fMSP) cells from fetal liver, fetal bone marrow, and cord blood. We hypothesize that a common fMSP exists in the developing fetus in multiple tissues. In this aim, each fMSP population will be fully characterized and compared with respect to in vitro growth characteristics, clonal expansion, multipotentiality, ability to support hematopoiesis, and expression of embryonal cell markers. 2) To define the developmental ontogeny of murine multipotent stromal progenitors. We hypothesize that fMSPs have a common site of origin in the fetus and migrate to hematopoietic and non-hematopoeitic tissues during development. To address this hypothesis, we will attempt to isolate fMSPs from specific hematopoietic and non-hematopoietic regions of the fetus prior to, and concurrent with, onset of hematopoiesis in the Aorto-Gonadal-Mesonephric region, fetal liver, and fetal bone marrow. 3) To assess fetal multipotent stromal progenitors in vivo in the murine in utero stem cell transplantation model. We will transplant optimized populations of transgenic GFP/male fMSPs into fetal recipients using techniques developed by our laboratory for intraperitoneal, or intravascular fetal injection. We will investigate homing, short and long term engraftment, and in vivo multipotentiality of fMSPs in the relatively unperturbed milieu of the fetal model. These studies should improve our understanding of the developmental biology of mesenchymal progenitors and should lead to a better understanding of their physiologic role in normal tissue remodeling and maturation and ultimately, what role, if any, they play in tissue repair and regeneration in response to injury or disease.